Desulfurization of fluid materials

ABSTRACT

A method for desulfurizing fluid materials, comprising reacting sulfur to be removed with a rare earth compound, thereby forming rare earth sulfides, oxysulfides or mixtures thereof. The reaction is conducted under conditions of low oxygen potential. Rare earth sulfides and oxysulfides can be reacted with oxygen to restore a capacity for desulfurization.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 174,024, filed July 31, 1980, now U.S. Pat. No. 4,397,683, which is a continuation-in-part of Ser. No. 031,531 filed Apr. 19, 1979, which in turn is a division of Ser. No. 838,945, filed Oct. 3, 1977 (now U.S. Pat. No. 4,161,400), which in turn is a continuation-in-part of Ser. No. 705,525, filed July 15, 1976 (now U.S. Pat. No. 4,084,960); and also is a continuation-in-part of copending Ser. No. 838,888, filed Oct. 3, 1977, which in turn is a continuation-in-part of Ser. No. 705,525, filed July 15, 1976 (now U.S. Pat. No. 4,084,960). Each of the aforementioned applications and patents is hereby incorporated by reference.

BACKGROUND AND SUMMARY OF THE INVENTION

This invention relates to methods of desulfurizing fluid materials and, more particularly, to a method of externally desulfurizing fluids such as molten iron and steel, stack gases, coal gases, coal liquification products, and the like using rare earth compounds, including such materials as rare earth oxides, rare earth fluorocarbonates or rare earth oxyfluorides, in an essentially dry process.

The term "rare earth", as used herein, includes the lanthanide rare earth elements having atomic numbers from 57 to 71, inclusive, and the element yttrium, atomic number 39, which is commonly found in rare earth concentrates and acts similarly to the rare earths in chemical separations.

As indicated above, this method is adapted to the desulfurization of essentially any fluid material. We shall, however, discuss the method in connection with two of the most pressing problems of desulfurization which industry presently faces; i.e., the desulfurization of molten iron and steel baths and the desulfurization of stack gases.

External desulfurization of molten iron and steel has been practiced for quite some time. It is a recognized, even necessary, practice in much of the iron and steel produced today. In current practices for the desulfurization of iron and steel it is common to add magnesium metal, magcoke, calcium oxide, calcium carbide or mixtures of calcium oxide and calcium carbide as the desulfurizing agent. Unfortunately, there are serious problems, as well as major cost items involved, in the use of all of these materials for desulfurization. Obviously, both calcium oxide and calcium carbide must be stored under dry conditions, since calcium oxide will hydrate and calcium carbide will liberate acetylene on contact with moisture. Magnesium is, of course, highly incendiary and must be carefully stored and handled. There are also further problems associated with the disposal of spent desulfurization slags containing unreacted calcium carbide.

We have found that these storage, material handling and disposal problems are markedly reduced by using rare earth compounds in a low oxygen content bath of molten iron or steel. The process is adapted to the desulfurization of pig iron or steel where carbon monoxide, evolved by the reaction where carbon is used as a deoxidizer, is diluted, either with an inert gas such as nitrogen or by vacuum degassing the melt, in order to reduce the oxygen potential and thereby increase the efficiency of the reaction by reducing the likelihood of forming oxysulfides. The principle may also be used for desulfurizing stack gases from boilers, etc., as shall be discussed in more detail hereafter.

In desulfurizing molten iron and steel in the practice of this invention, it is preferable to follow the steps of reacting rare earth oxide, rare earth oxyfluorides, rare earth fluorocarbonates and mixtures thereof (including bastnasite concentrates), in the presence of a deoxidizing agent, with the sulfur to be removed, to form one of the group consisting of rare earth sulfide and rare earth oxysulfide and mixtures thereof.

Preferably, hot metal is treated in a ladle or transfer car with rare earth compounds, by the simple addition and mixing of the rare earth compounds, by an injection technique in which the rare earth compounds are injected into the molten bath in a carrier gas such as argon or nitrogen, or by the use of an "active lining"; i.e., a rare earth compound lining in the vessel. In any case, the chemical reactions involved may be shown as follows, where the term RE indicates "rare earth":

    2CeO.sub.2(s) +[C]=Ce.sub.2 O.sub.3(s) +CO.sub.(g)         ( 1)

    RE.sub.2 O.sub.3(s) +[C]+[S].sub.1w/o =RE.sub.2 O.sub.2 S.sub.(s) +CO.sub.(g)                                               ( 2)

and

    RE.sub.2 O.sub.2 S.sub.(s) +2[C]+2[S].sub.1w/o =RE.sub.2 S.sub.3 +2CO.sub.(g)                                              ( 3)

The product sulfide or oxysulfide can either be fixed in an `active` lining or removed by flotation and absorbed into the slag cover and vessel lining, depending upon the process used for introducing the rare earth compound.

The products of desulfurization of carbon saturated iron with rare earth oxides is dependent on the partial pressure of CO, pCO, and the Henrian sulfur activity in the metal, h_(S). Using cerium as the representative rare earth, the following standard free energy changes and the equilibrium constants at 1,500° C. for different desulfurization reactions can be calculated from thermodynamic data in the literature:

    __________________________________________________________________________     REACTION         ΔG° cal.                                                                   K.sub.1773                                           __________________________________________________________________________     2CeO.sub.2(s) + [C] = Ce.sub.2 O.sub.3(s) + CO.sub.(g)                                          66000 - 53.16T                                                                          pCO = 3041                                           Ce.sub.2 O.sub.3(s) + [C] + [S].sub.1w/o =                                                      18220 - 26.43T                                                                          pCO/h.sub.S = 3395                                   Ce.sub.2 O.sub.2 S.sub.(s) + CO.sub.(g)                                        Ce.sub.2 O.sub.2 S.sub.(s) + [C] + 2[S].sub.1w/o =                                              66180 - 39.86T                                                                          p.sup.2 CO/h.sub.S.sup.2 = 3.6                       Ce.sub.2 S.sub.3(s) + 2CO.sub.(g)                                              3/2Ce.sub.2 O.sub.2 S.sub.(s) + 3[C] + 5/2[S].sub.1w/o =                                        127050 - 72.1T                                                                          p.sup.3 CO/h.sub.S.sup.5/2 = 1.25                    Ce.sub.3 S.sub.4(s) + 3CO.sub.(g)                                              Ce.sub.2 O.sub.2 S.sub.(s) + 2[C] + [S].sub.1w/o =                                              120,860 - 61.0T                                                                         p.sup. 2 CO/h.sub.S = .027                           2CeS.sub.(s) + 2CO.sub.(g)                                                     C.sub.(s) + 1/2O.sub.2(g) = CO.sub.(g)                                                          -28200 - 20.16T                                                                         pCO/p.sup.1/2 O.sub.2 = 7.6 × 10.sup.-7        1/2S.sub.2(g) = [S].sub.1w/o                                                                    -31520 + 5.27T                                                                          h.sub.S /1.sup.1/2 S.sub.2 = 5.4 ×                                       10.sup.2                                             __________________________________________________________________________

The thermodynamics of desulfurization with lanthanum oxide, La₂ O₃, are similar although, in this case, LaO₂ is unstable and there will be no conversion corresponding to CeO₂ →Ce₂ O₃.

In the case of desulfurization of gases, such as stack gases, assume the following gas composition at 1,000° C.:

    ______________________________________                                         Component    Vol. %                                                            ______________________________________                                         CO.sub.2     16                                                                CO           40                                                                H.sub.2      40                                                                N.sub.2       4                                                                H.sub.2 S      0.3                                                                          (200 grains/100 ft..sup.3)                                        ______________________________________                                    

This equilibrium gas composition is represented by point A on the diagram illustrated as FIG. 6 where CO/CO₂ =2.5 and H₂ /H₂ S=133. This point lies within the Ce₂ O₂ S phase field and at constant CO/CO₂ desulfurization with Ce₂ O₃ will take place up to point B. At point B, H₂ /H₂ S≃10⁴ and the concentration of H₂ S is 0.004 vol.% (˜3 grains/100 ft.³). Beyond this point, desulfurization is not possible.

The basic theory for this invention is supported by the standard free energies of rare earth compounds likely to be involved. Examples of these appear in Table I which follows:

                                      TABLE I                                      __________________________________________________________________________     Standard Free Energies of Formation of Some Rare Earth Compounds:              ΔG° = X - YT cal/g.f.w.                                                                             Estimated                                    Reaction         X     Y   Temp. (°K.).                                                                   Error (Kcal)                                 __________________________________________________________________________     CeO.sub.2(s) = Ce.sub.(1) + O.sub.2(g)                                                          259,900                                                                              49.5                                                                               1071-2000                                                                             ±3                                        Ce.sub.2 O.sub.3(s) = 2Ce.sub.(1) + 3/2O.sub.2(g)                                               425,621                                                                              66.0                                                                               1071-2000                                                                             ±3                                        La.sub.2 O.sub.3(s) = 2La.sub.(1) + 3/2O.sub.2(g)                                               428,655                                                                              68.0                                                                               1193-2000                                                                             ±3                                        CeS.sub.(s) = Ce.sub.(1) + 1/2S.sub.2(g)                                                        132,480                                                                              24.9                                                                               1071-2000                                                                             ±2                                        Ce.sub.3 S.sub.4(s) = 3Ce.sub.(1) + 2S.sub.2(g)                                                 483,180                                                                              98.2.sup.( *.sup.)                                                                 1071-2000                                                                              ±10                                      Ce.sub.2 S.sub.3(s) = 2Ce.sub.(1) + 3/2S.sub.2(g)                                                  351,160.sup.( *.sup.)                                                             76.0.sup.( *.sup.)                                                                 1071-2000                                                                              ±10                                      LaS.sub.(s) = La.sub.(1) + 1/2S.sub.2(g)                                                        123,250                                                                              25.3                                                                               1193-2000                                                                             ±6                                        Ce.sub. 2 O.sub.2 S.sub.(s) = 2Ce.sub.(1) + O.sub.2(g) + 1/2S.sub.2(g)                          410,730                                                                              65.0                                                                               1071-2000                                                                              ±15                                      La.sub.2 O.sub.2 S.sub.(s) = 2La.sub.(s) + O.sub.2(g) + 1/2S.sub.2(g)                              407,700.sup.( *.sup.)                                                             65.0.sup.( *.sup.)                                                                 1193-2000                                                                              ±15                                      __________________________________________________________________________      .sup.( *.sup.) Estimated                                                 

The three phase equilibria at 1273° K. for the Ce--O--S System is set out in Table II as follows:

                                      TABLE II                                     __________________________________________________________________________     Ce--O--S System                                                                Three Phase Equilibria at 1273° K.                                      REACTION            ΔG° cal                                                                    K.sub.1273                                        __________________________________________________________________________     Ce.sub.2 O.sub.3(s) + 1/2S.sub.2(g) = Ce.sub.2 O.sub.2 S.sub.(s) +             1/2O.sub.2(g)       14890 - 1.0T                                                                            (pO.sub.2 /pS.sub.2).sup.1/2 = 4.6 ×                                     10.sup.-3                                         Ce.sub.2 O.sub.2 S.sub.(s) + 1/2S.sub.2(g) = 2CeS.sub.(s)                                          145770 - 15.2T                                                                          pO.sub.2 /p.sup.1/2 S.sub.2 = 2.0 ×                                      10.sup.-22                                        3Ce.sub.2 O.sub.2 S.sub.(s) + 5/2S.sub. 2(g) = 2Ce.sub.3 S.sub.4(s) +          30.sub.2(g)         265830 + 1.4T                                                                           p.sup.3 O.sub.2 /p.sup.5/2 S.sub.2 = 1.1                                       × 10.sup.-46                                Ce.sub.2 O.sub.2 S.sub.(s) + S.sub.2(g) = Ce.sub.2 S.sub.3                                         59570 + 11.0T                                                                           pO.sub.2 /pS.sub.2 = 2.3 ×                                               10.sup.-13                                        Ce.sub.3 S.sub.4(s) = 3CeS.sub.(s) + 1/2S.sub.2(g)                                                 85740 - 23.5T                                                                           p.sup.1/2 S.sub.2 = 2.5 × 10.sup.-10        2Ce.sub.2 S.sub.3(s) = 2Ce.sub.3 S.sub.4(s) + 1/2 S.sub.2(g)                                       87120 - 31.6T                                                                           p.sup.1/2 S.sub.2 = 8.9 × 10.sup.-8         CO.sub.(g) + 1/2O.sub.2(g) = CO.sub.2(g)                                                           -67500 + 20.75T                                                                         pCO.sub.2 /(pCO.p.sup.1/2 O.sub.2) = 1.1                                       × 10.sup.7                                  H.sub.2(g) + 1/2S.sub.2(g) = H.sub.2 S.sub.(g)                                                     -21580 + 11.80T                                                                         pH.sub.2 S/(pH.sub.2.p.sup.1/2 S.sub.2) =                                      13.4                                              H.sub.2(g) + 1/2O.sub.2(g) = H.sub.2 O.sub.(g)                                                     -58900 + 13.1T                                                                          pH.sub.2 O(pH.sub.2.p.sup.1/2 O.sub.2) = 1.8                                   × 10.sup.7                                  __________________________________________________________________________

Typical calculations of energy changes involved in the systems involved in this invention are as follows:

    ______________________________________                                         S.sub.2(g) + Ce.sub.2 O.sub.2 S.sub.(s) = Ce.sub.2 S.sub.3(s)                  + O.sub.2(g)                                                                   Ce.sub.2 S.sub.3(s) = 2Ce.sub.(l) + 3/2S.sub.2(g) : ΔG° =         351160 - 76.0T cal                                                             Ce.sub.2 O.sub.2 S.sub.(s) = 2Ce.sub.(l) + O.sub.2(g) + 1/2S.sub.2(g) :        ΔG° = 410730 - 65.0T cal                                          Ce.sub.2 O.sub.2 S.sub.(s) + S.sub.2(g) = Ce.sub.2 S.sub.3(s)                  + O.sub.2(g) : ΔG° = 59570 + 11.0T cal                            @ 1273° K. ΔG° = 73573 cal and pO.sub.2 /pS.sub.2 =        2.33 × 10.sup.-13                                                        Ce.sub.2 O.sub.3(s) + 1/2S.sub.2(g) = Ce.sub.2 O.sub.2 S + 1/2O.sub.2(g)       Ce.sub.2 O.sub.3(s) = 2Ce.sub.(l) + 3/2O.sub. 2(g) : ΔG° =        425621 - 66.0T cal                                                             Ce.sub. 2 O.sub.2 S.sub.(s) = 2Ce.sub.(l) + O.sub.2(g) + 1/2S.sub.2(g) :       ΔG° = 410730 - 65.0T cal                                          Ce.sub.2 O.sub.3(s) + 1/2S.sub.2(g) = Ce.sub.2 O.sub.2 S.sub.(s) +             1/2O.sub.2(g) : G° = 14891 - 1.0T cal                                   @ 1273° K. ΔG° = 13618 cal and (pO.sub.2 /pS.sub.2).su     p.1/2  = 4.6 × 10.sup.-3                                                 Ce.sub.2 O.sub.2 S.sub.(s) + 1/2S.sub.2(g) = 2CeS.sub.(s) + O.sub.2(g)         Ce.sub.2 O.sub.2 S.sub.(s) = 2Ce.sub.(l) + 1/2S.sub.2(g) + O.sub.2(g) :        ΔG° = 410730 - 65.0T cal                                          2CeS.sub.(s) = 2Ce.sub.(l) + S.sub.2(g) : ΔG° = 264960 -          4.98T cal                                                                      Ce.sub.2 O.sub.2 S.sub.(s) + 1/2S.sub.2(g) = 2CeS.sub.(s) + O.sub.2(g) :       ΔG°  = 145770 - 15.2T cal                                         @ 1273° K. ΔG° = 126420 cal. and pO.sub.2 /p.sup.1/2       S.sub.2 = 1.96 × 10.sup.-22                                              3Ce.sub.2 O.sub.2 S.sub.(s) + 5/2S.sub.2(g) = 2Ce.sub.3 S.sub.4(s) +           3O.sub. 2(g)                                                                   2Ce.sub.3 S.sub.4(s) = 6Ce.sub.(l) + 4S.sub.2(g) : ΔG° =          966360 - 196.4T cal                                                            3Ce.sub.2 O.sub.2 S.sub.(s) = 6Ce.sub.(l) + 3O.sub. 2(g) + 3/2S.sub.2(g)       : ΔG° = 1232190 -                                                 195.0T cal                                                                     3Ce.sub.2 O.sub.2 S.sub.(s) + 5/2S.sub.2(g) = 2Ce.sub.3 S.sub.4(s) +           3O.sub. 2(g) : ΔG° = 265830 +                                     1.4T cal @ 1273° K. ΔG° = 267612 cal and p.sup.3           O.sub.2 /p.sup.5/2 S.sub.2 = 1.12 ×                                      10.sup.-46                                                                     Ce.sub.3 S.sub.4(s) = 3CeS.sub.(s) + 1/2S.sub.2(g)                             Ce.sub.3 S.sub.4(s) = 3Ce.sub. (l) + 2S.sub.2(g) : ΔG° =          48318 - 98.2T cal                                                              3CeS.sub.(s) = 3Ce.sub.(l) + 3/2S.sub.2(g) : ΔG° = 397,440 -      74.7T cal                                                                      Ce.sub.3 S.sub.4(s) = 3CeS.sub.(s) + 1/2S.sub.2(g) : ΔG° =        85740 - 23.5T cal                                                              @ 1273° K. ΔG° = 55824 cal p.sup.1/2 S.sub.2 = 2.6         × 10.sup.-10                                                             3Ce.sub.2 S.sub.3(s) = 2Ce.sub.3 S.sub.4(s) + 1/2S.sub.2(g)                    2Ce.sub.3 S.sub.4(s) = 6Ce.sub.(l) + 4S.sub.2(g) : ΔG° =          966360 - 196.4T cal                                                            3Ce.sub.3 S.sub.3(s) = 6Ce.sub.(l) + 9/2S.sub.2(g) : ΔG° =        1053480 - 228.0T cal                                                           3Ce.sub.2 S.sub.3(s) = 2Ce.sub.3 S.sub.4(s) + 1/2S.sub.2(g) :                  ΔG°  = 87120 - 31.6T cal                                          @ 1273° K. ΔG° = 468893 cal and p.sup.1/2 S.sub.2 =        8.9 × 10.sup.-9                                                          ______________________________________                                         H.sub.2(g) + 1/2S.sub.2(g) = H.sub.2 S.sub.(g)                                 H.sub.2(g) + 1/2S.sub.2(g) = H.sub.2 S.sub.(g) : ΔG° =            -21580 + 11.80T cal                                                            @ 1273° K. ΔG° = -6559 and pH.sub.2 S/(pH.sub.2.p.sup.     1/2 S.sub.2) = 13.4                                                            ______________________________________                                                pH.sub.2 /pH.sub.2 S                                                                   log pS.sub.2                                                    ______________________________________                                                1        -2.25                                                                 10.sup.2                                                                                -6.25                                                                 10.sup.4                                                                               -10.25                                                                 10.sup.6                                                                               -14.25                                                                 10.sup.8                                                                               -18.25                                                                 .sup. 10.sup.10                                                                        -22.25                                                                 .sup. 10.sup.12                                                                        -26.25                                                          ______________________________________                                         H.sub.2(g) + 1/2O.sub.2(g) = H.sub.2 O.sub.(g)                                 H.sub.2(g) + 1/2 O.sub.2(g) = H.sub.2 O.sub.(g) : ΔG° =           -58900 + 13.1T cal                                                             @ 1273°  K. ΔG° = -42223 cal and (pH.sub.2 /pH.sub.2       O) p.sup.1/2 O.sub.2 =                                                         5.6 × 10.sup.-8                                                          ______________________________________                                                pH.sub.2 /pH.sub.2 O                                                                   log pO.sub.2                                                    ______________________________________                                                .sup. 10.sup.-4                                                                         -6.5                                                                  .sup. 10.sup.-2                                                                        -10.5                                                                  1       -14.5                                                                  10.sup.2                                                                               -18.5                                                                  10.sup.4                                                                               -22.5                                                                  10.sup.6                                                                               -26.5                                                                  10.sup.8                                                                               -30.5                                                           ______________________________________                                         Co.sub.(g) + 1/2 O.sub.2(g) = CO.sub.(g)                                       Co.sub.(g) 1/2 O.sub.2(g) = CO.sub.2(g) : ΔG° = -67500 +          20.75T cal                                                                     @ 1273° K. ΔG° = -41085 and pCO.sub.2 /(pCO.p.sup.1/2      O.sub.2) = 1.1 × 10.sup.7                                                ______________________________________                                                pCO/pCO.sub.2                                                                          log pO.sub.2                                                    ______________________________________                                                .sup. 10.sup.-4                                                                         -6.1                                                                  .sup. 10.sup.-2                                                                        -10.1                                                                  1       -14.1                                                                  10.sup.2                                                                               -18.1                                                                  10.sup.4                                                                               -20.1                                                                  10.sup.6                                                                               -24.1                                                                  10.sup.8                                                                               -30.1                                                           ______________________________________                                    

DETAILED DESCRIPTION

In the foregoing general description of this invention, certain objects, purposes and advantages have been outlined. Other objects, purposes and advantages of this invention will be apparent, however, from the following description and the accompanying drawings in which:

FIG. 1 is a phase stability diagram for cerium oxide, oxysulfide and sulfides in hot metal at 1500° C.;

FIGS. 2a and 2b show Ce₂ S₃ and Ce₂ O₂ S layers on a pellet of CeO₂ ;

FIG. 3 is a graph of the theoretical CeO₂ requirement for sulfur removal in hot metal;

FIG. 4 is a graph showing the volume of nitrogen required to produce a given partial pressure of CO;

FIG. 5 is a graph showing the CeO₂ requirements as a function of partial pressure of CO;

FIG. 6 is a stability diagram for stack gas systems treated according to this invention; and

FIG. 7 is a graph showing hot metal sulfur content as a function of time for the practice of the invention as described in Example 1.

Referring back to the discussion of free energy set out above, it is clear that these free energy changes may be used to determine the fields of stability of Ce₂ O₃, Ce₂ O₂ S, Ce₂ S₃, Ce₃ S₄ and CeS in terms of the partial pressure of CO and the Henrian sulfur activity of the melt at 1500° C. The resultant stability diagram is shown in FIG. 1, the boundaries between the phase fields being given by the following relationships:

    ______________________________________                                         BOUNDARY        EQUATION                                                       ______________________________________                                         Ce.sub.2 O.sub.3 --Ce.sub.2 O.sub.2 S                                                          log pCO = log h.sub.S + 3.53                                   Ce.sub.2 O.sub.2 S--Ce.sub.2 S.sub.3                                                           log pCO = log h.sub.S + 0.28                                   Ce.sub.2 O.sub.2 S--Ce.sub.3 S.sub.4                                                           log pCO = 0.83 log h.sub.S + 0.03                              Ce.sub.2 O.sub.2 S--CeS                                                                        log pCO = 0.5 log h.sub.S - 0.79                               Ce.sub.2 S.sub.3 --Ce.sub.3 S.sub.4                                                            log h.sub.S = -1.47                                            Ce.sub.3 S.sub.4 --CeS                                                                         log h.sub.S = -2.45                                            ______________________________________                                    

The phase fields in FIG. 1 are also shown in terms of the Henrian activity of oxygen, h_(O), and the approximate [w/o S] in the iron melt using an activity coefficient f_(S) ≃5.5 for graphite saturated conditions.

The coordinates of the points B, C, D and E on the diagram are given below:

    ______________________________________                                         COORDI-                                                                        NATES    B         C         D       E                                         ______________________________________                                         pCO atm. 9.8 × 10.sup.-3                                                                    6.5 × 10.sup.-2                                                                    1.0     1.0                                       h.sub.S  3.5 × 10.sup.-3                                                                    3.4 × 10.sup.-2                                                                    5.3 × 10.sup.-1                                                                  2.9 × 10.sup.-4                     Approx.  6.4 × 10.sup.-4                                                                    6.2 × 10.sup.-3                                                                    9.6 × 10.sup.-2                                                                  5.3 × 10.sup.-5                     [w/o S]                                                                        ______________________________________                                    

The points B and C represent simultaneous equilibria between the oxysulfide and two sulfides at 1500° C. These univariant points are only a function of temperature. The points E and D represent the minimum sulfur contents or activities at which oxysulfide and Ce₂ S₃ can be formed, respectively, at pCO=1 atm. Thus, carbon saturated hot metal cannot be desulfurized by oxysulfide formation below h_(S) ≃2.9×10⁻⁴ ([w/o S]≃5.3×10⁻⁵) at pCO=1 atm. However, lower sulfur levels may be attained by reducing the partial pressure of CO.

The conversion of CeO₂ →Ce₂ O₃ →Ce₂ O₂ S→Ce₂ S₃ is illustrated in FIGS. 2a and 2b which show Ce₂ S₃ and Ce₂ O₂ S layers on a pellet of CeO₂ (which first transformed to Ce₂ O₃) on immersion in graphite saturated iron at ˜1600° C., initially containing 0.10 w/o S, for 10 hours. The final sulfur content was ˜0.03 w/o S and the experiment was carried out under argon, where pCO<<1 atm.

The conversion of the oxide to oxysulfide and sulfide is mass transfer controlled and, as in conventional external desulfurization with calcium carbide, vigorous stirring will be required for the simple addition process and circulation of hot metal may be required in the `active` lining process.

From FIG. 1 it is apparent that the external desulfurization of graphite saturated iron is thermodynamically possible using RE oxides. For example, the diagram indicates that hot metal sulfur levels of ˜0.5 ppm (point E) can be achieved by cerium oxide addition even at pCO=1 atm. Desulfurization in this case will take place through the transformation sequence CeO₂ →Ce₂ O₃ →Ce₂ O₂ S which requires 2 moles of CeO₂ to remove 1 gram atom of sulfur. The efficiency of sulfur removal/lb. CeO₂ added can, however, be greatly increased by the formation of sulfides. 1 mole CeO₂ is required per gram atom of sulfur for CeS formation and 2/3 moles CeO₂ for Ce₂ S₃ formation. The theoretical CeO₂ requirements for the removal of sulfur in hot metal for the various desulfurization products are given below and expressed graphically in FIG. 3. The term THM is used herein as an abbreviation for "ton of hot metal".

    ______________________________________                                         PRO-  lb CeO.sub.2 /0.01                                                       DUCT  w/o S.THM  ft.sup.3 CO/lb CeO.sub.2                                                                   ft.sup.3 CO/0.01 w/o S.THM                        ______________________________________                                         Ce.sub.2 O.sub.2 S                                                                    2.15      2.1         4.5                                               CeS   1.1        4.2         4.5                                               Ce.sub.3 S.sub.4                                                                     0.8        4.2         3.4                                               Ce.sub.2 S.sub.3                                                                     0.7        4.2         3.0                                               ______________________________________                                    

The volume of carbon monoxide produced in ft³ CO/lb CeO₂ and ft³ CO/0.01 w/o S.THM are also given in the above table for each desulfurization product. For efficient desulfurization the partial pressure of carbon monoxide should be sufficiently low to avoid oxysulfide formation. For example, FIG. 1 shows that oxysulfide will not form in a graphite saturated melt until [w/o S]<0.01 when pCO≃0.1 atm. It will form however when [w/o S]≃0.10 at pCO=1 atm. Thus, by reducing the pCO in the desulfurization process at 0.1 atm, hot metal can be desulfurized to 0.01 w/o S with a CeO₂ addition of 0.72 lb/0.01 w/o S removed for each ton hot metal.

The choice of the method of reducing the partial pressure of carbon monoxide depends on economic and technical considerations. However, in an injection process, calculations can be made for the volume of injection gas, say nitrogen, required to produce a given pCO. Thus:

    V.sub.N.sbsb.2 =V.sub.CO (1-pCO)/pCO

where

V_(CO) is the scf of CO formed/lb CeO₂ added

V_(N).sbsb.2 is the scf of N₂ required/lb CeO₂ added and

pCO is the desired partial pressure of CO in atm.

The results of these calculations for Ce₂ S₃ formation are shown in FIG. 4, which also shows the [w/o S] in equilibrium with Ce₂ S₃(s) as a function of pCO. From this figure it is apparent that the volume of N₂ /lb CeO₂ required to form Ce₂ S₃ is excessive and if an injection process were used a balance would have to be struck between sulfide and oxysulfide formation. When, for example, hot metal is to be desulfurized from 0.05 to 0.01 w/o S at pCO=0.2 atm., ˜16 scf N₂ /lb CeO₂ would be required for Ce₂ S₃ formation and the sulfur content would drop to 0.02 w/o. The remaining 0.01 w/o S would be removed by oxysulfide formation. From FIG. 3, it can be seen that ˜2 lbs of CeO₂ /THM would be required for Ce₂ S₃ formation and 2 lbs for Ce₂ O₂ S formation giving a total requirement of 4 lbs CeO₂ /THM.

Calculations similar to the one above have been used to construct FIG. 5 where the CeO₂ requirements in lbs/THM are shown as a function of pCO.

When large volumes of nitrogen are used in an injection process the heat carried away by the nitrogen, as sensible heat, is not large but the increased losses by radiation may be excessive. Injection rates with calcium carbide for example are in the order of 0.1 scf N₂ /lb CaC₂.

Vacuum processing is an alternative method of reducing the partial pressure of carbon monoxide. This is impractical in hot metal external desulfurization but not in steelmaking (see below).

Still another alternative approach to external desulfurization using rare earth compounds is the use of active linings which would involve the `gunning` or flame-spraying of hot metal transfer car linings with rare earth compounds. Here the compounds would transform to oxysulfides during the transfer of hot metal from the blast furnace to the steelmaking plant, and the oxide would be regenerated by atmospheric oxidation when the car was emptied. It is estimated that for a 200 ton transfer car, conversion of a 2 mm layer (˜0.080") of oxide to oxysulfide would reduce the sulfur content of the hot metal by ˜0.02 w/o S. This process has the following advantages:

(1) continuous regeneration of rare earth oxide by atmospheric oxidation when the car is empty,

(2) reaction times would be in the order of hours,

(3) the absence of a sulfur rich desulfurization slag, and

(4) the absence of suspended sulfides in the hot metal. The mechanical integrity and the life of an "active" lining is, of course, critical and some pollution problems may be associated with oxide regeneration by atmospheric oxidation.

With regard to steelmaking applications, vacuum desulfurization could be carried out by an "active" lining in the ASEA-SKF process and circulation vacuum degassing processes.

Although a large portion of this discussion is concerned with oxides of single rare earths, it should be noted that other compositions containing mixtures of rare earths and varying amounts of non-rare earth elements and compounds can be used in the practice of the invention. For example, such low cost materials as concentrates of bastnasite, a rare earth fluorocarbonate ore, or the rare earth oxyfluorides formed by calcining bastnasite concentrates, are useful. Typical approximate weight percentage compositions for Bastnasite 4000 concentrates and acid-leached Bastnasite 4010 concentrates, produced by Molycorp, Inc., a subsidiary of Union Oil Company of California, are shown in Table III. Also shown are the calculated compositions for calcined products of the concentrates, after removal of the loss on ignition. A calcined Bastnasite 4000 is denoted "4000C", while calcined Bastnasite 4010 is denoted "4100".

                  TABLE III                                                        ______________________________________                                         Composition of Bastnasite Concentrates                                         ______________________________________                                                      Bastnasite Concentrate                                            Component      4000    4000C    4010  4100                                     ______________________________________                                         Contained RE*  55-60   72-79    68-72 85-90                                    (as oxide)                                                                     SrO            6.0     7.9      1.0   1.2                                      CaO            5.0     6.6      0.4   0.5                                      BaO            1.5     2.0      1.8   2.2                                      F              4.0     5.3      5.0   6.0                                      SiO.sub.2      0.4     0.5      0.4   0.6                                      Fe.sub.2 O.sub.3                                                                              0.5     0.7      0.5   0.6                                      P.sub.2 O.sub.5                                                                               0.9     1.2      1.0   1.2                                      MgO, Na.sub.2 O, K.sub.2 O (each)                                                             <0.5    <0.5     <0.5  <0.5                                     ThO.sub.2      <0.1    <0.1     <0.1  <0.1                                     Loss on ignition                                                                              24.0    --       20.0  --                                       (primarily CO.sub.2)                                                           ______________________________________                                         Oxide       % of total RE                                                      ______________________________________                                         CeO.sub.2   48-50                                                              La.sub.2 O.sub.3                                                                           32-34                                                              Nd.sub.2 O.sub.3                                                                           13-14                                                              Pr.sub.6 O.sub.11                                                                          4-5                                                                Sm.sub.2 O.sub.3                                                                           0.5                                                                Gd.sub.2 O.sub.3                                                                           0.2                                                                Eu.sub.2 O.sub.3                                                                           0.2                                                                Others      0.2                                                                ______________________________________                                          *Contained RE (as oxides)                                                

The invention is further illustrated by the following examples which are illustrative of various aspects of the invention, and are not intended as limiting the scope of the invention as defined by the appended claims.

EXAMPLE 1

An experiment demonstrating the use of rare earth compositions for desulfurizing molten iron is conducted, utilizing the "active lining" technique.

Crucibles of about 3 inches outside diameter and about 8 inches height are prepared by hand ramming lining material into the bottom, to a depth of about 0.5 inches. Walls are then constructed by ramming the lining material between the crucible wall and a cylindrical graphite former. With the former in place, the lining is sintered by heating to about 1600° C. in an induction furnace and maintaining the temperature for about one hour. Ventilation holes in the former permit any gases to escape during sintering. After removal of the former, the rare earth lining has a thickness of about 0.25 inches.

Cerium oxide, CeO₂, and two calcined bastnasite concentrates are used as lining materials. The two bastnasites, produced by Molycorp, Inc., are No. 4000C (unleached, calcined bastnasite) and No. 4100 (acid leached, calcined bastnasite).

The experimental procedure is as follows: a cylindrical ingot of graphite-saturated iron is placed in a lined crucible and melted in an induction furnace. Upon reaching the experimental temperature, about 1450° C., iron sulfide is added to establish the initial sulfur content. The melt is held at the experimental temperature and suction samples are taken at time intervals for sulfur analysis. After the experiment, melts are either solidified in situ or cast. Following casting, the crucibles can be re-used for desulfurization, due to the regenerative effect of air contact on the lining material.

Results are shown in FIG. 7, where sulfur content is given as a function of time for melts contained within CeO₂, Bastnasite 4000C and Bastnasite 4100 lined crucibles.

All linings appear to be mechanically sound after use, and one CeO₂ lining is re-used with the results shown in FIG. 7. Some evidence of lining flaking is noted in the more refractory CeO₂ and Bastnasite 4100 linings, but, since Bastnasite 4000C appears to soften at about 1500° C., it is therefore not as susceptible to failure by spalling or flaking. In cases where the desulfurized iron is cast, the reacted lining appears to react and fume upon contact with air, probably because of regeneration reactions.

Subsequent experiments for desulfurizing Fe-C-Si-Mn-S melts containing about 1% by weight Si, however, indicate that the rate of sulfur removal is significantly lower for re-used linings, probably due to deposition of a surface layer of silica on the lining during regeneration. This layer presumably decreases accessibility of the rare earth compound for reaction with sulfur.

EXAMPLE 2

An experiment is performed to illustrate the use of rare earth compounds for desulfurizing gases.

Pellets of Bastnasite 4100 are prepared by mixing the powder with 10-20 percent by weight of water and pelletizing in a 40 cm diameter rubber tire pelletizer, revolving at 60 rpm, for about three minutes. Pellet size is controlled by the water content; 0.1 cm pellets result from about 10% water addition and 1 cm diameters are produced by about 20% water content in the mixture.

The pellets are dried in an oven at about 75° C. for about 5 hours, then sintered in air at about 1200° C. for approximately 7 hours. Those having a diameter of about 0.5 cm are selected for subsequent use.

Cerium in the pellets is reduced to the trivalent state by contact with hydrogen for about two hours at 800° C., to facilitate desulfurization reactions involving conversion to oxysulfide under reducing conditions.

Reduced pellets are placed in a silica reactor tube to form a fixed bed measuring approximately 4 inches diameter and 6 inches length. A thermocouple is inserted into the bed, and the tube is placed inside a tube furnace. A sulfur-containing gas, representative of coal gasification processes and containing 1.0% H₂ S, 33.0% H₂, 10.8% CO₂ and 56.2% CO by volume, is passed through the bed at a rate of about 0.5 l/min. The H₂ S content of the output gas is measured as a function of temperature and time by drawing 100 ml samples through a disposable H₂ S Drager tube, which contains lead acetate, and reading the approximate H₂ S concentration in ppm by the length of black lead sulfide which forms in the tube.

Results from the experiment are shown in Table IV. In one case, the pellets used for desulfurization at 850° C. are oxidized in situ by passing air through the bed at about 0.5 l/min. for about one hour. The pellets are reduced at about 800° C. in hydrogen and re-used for desulfurization at 850° C., as shown in the column marked "Regen-850° C.".

                  TABLE IV                                                         ______________________________________                                         Desulfurization of 10,000 ppm H.sub.2 S Gas                                    Time  H.sub.2 S Content of Outlet Gas (ppm)                                     (min.)                                                                              250° C.                                                                         450° C.                                                                         55° C.                                                                         850° C.                                                                       Regen. -850° C.                      ______________________________________                                          5     <1     <1      <1     <1    <1                                          10    1600    <1      <1     <1    <1                                          15    8000    <1      <1     2     <1                                          20    --      <1      2      2     1300                                        25    --      <1      3      2     2000                                        30    --       7      5      4     1000                                        35    --       60     19     6     250                                         40    --      --      55     7     250                                         45    --      1300    1100   10    250                                         50    --      2000    1600   13    200                                         55    --      5000    3200   14    200                                         60    --      --      4500   --    200                                         ______________________________________                                    

EXAMPLE 3

An experiment similar to that of Example 2 is performed, using pellets of Bastnasite 4000C for desulfurizing gases.

Pellets, prepared as in the preceding example, except sintered at a temperature of either 1100° C. or 1300° C., and reduced in hydrogen at 1200° C. for about 24 hours, are formed into a bed and treated as before. Two gases are used: the first is the 1.0% H₂ S by volume (10,000 ppm) gas of Example 2, the second is a mixture containing 2.27% H₂ S (22,700 ppm) and 97.73% H₂ by volume.

The results are shown in Table V, which correlates sulfur removal with both desulfurization conditions and preparation differences between the pellets used. The column marked "Regenerated" contains data obtained after treating the used 1300° C. pellets with air for four hours at about 1200° C., reducing with hydrogen, and repeating the previous desulfurization test.

During initial desulfurization, a layer of reaction product about 0.5 mm thick forms on the pellet surfaces. Upon regeneration, a certain amount of the surface layer exfoliates. It is possible that the improved sulfur capacity of the regenerated material may be due, in part, to exposure of the previously unreacted pellet core.

                  TABLE V                                                          ______________________________________                                         Desulfurization of Gases                                                       ______________________________________                                                                          Regen-                                        Pellet Type   Initial Use        erated                                        ______________________________________                                         Sintering (°C./hours)                                                                 1100/5  1100/5     1300/7                                                                               1300/7                                  H.sub.2 S in Feed Gas (ppm)                                                                  10,000  10,000     22,700                                                                               22,700                                  Feed Gas Flow (l/min.)                                                                       0.5     0.35       1.06  1.06                                    Bed Temp. (°C.)                                                                         850     1000       1000                                                                                 1000                                  ______________________________________                                         Time (minutes)                                                                               H.sub.2 S content of Outlet Gas (ppm)                            ______________________________________                                         5             <1       10        100    60                                     10            <1       10        400   100                                     20            <1      <30        480   100                                     60            <2      <30        560   100                                     80            <2      <30        580   100                                     100            8      <30        590   100                                     120            20     <30        590   100                                     140            25     <30        590   100                                     160            35     <30        590   100                                     180           100     <30        600   100                                     200           --      <30        700   100                                     220           --      <30        900   100                                     230           --        <30.sup.++                                                                              1600  --                                      ______________________________________                                          .sup.++ No change in H.sub. 2 S content after 24 hours.                  

Various embodiments and modifications of this invention have been described in the foregoing description and examples, and further modifications will be apparent to those skilled in the art. Such modifications are included within the scope of the invention as defined by the following claims. 

We claim:
 1. A method for decreasing the concentration of reactive sulfur in a fluid material, comprising reacting at least a portion of the sulfur with a compound containing a rare earth element, under reducing conditions.
 2. The method as described in claim 1 wherein the compound is selected from the group consisting of rare earth oxides, rare earth fluorocarbonates, rare earth oxyfluorides, and mixtures thereof.
 3. The method as described in claim 2 wherein rare earth fluorocarbonates comprise concentrates of bastnasite.
 4. The method as described in claim 2 wherein rare earth oxyfluorides comprise roasted concentrates of bastnasite.
 5. The method as described in claim 2 wherein rare earth oxyfluorides comprise acid leached, roasted concentrates of bastnasite.
 6. The method as described in claim 1 wherein a low oxygen potential is obtained by introducing an inert gas.
 7. The method as described in claim 1 wherein a low oxygen potential is obtained by applying a partial vacuum.
 8. The method as described in claim 1 wherein a low oxygen potential is obtained by introducing a deoxidizing agent.
 9. The method as described in claim 8 wherein a deoxidizing agent is carbon.
 10. The method as described in claim 9 wherein carbon monoxide, produced from the oxidation of carbon, is maintained at a partial pressure below about 0.1 atmosphere.
 11. The method as described in claim 1 wherein the fluid material is a liquid.
 12. The method as described in claim 11 wherein the liquid comprises molten metal.
 13. The method as described in claim 12 wherein the metal is iron.
 14. The method as described in claim 1 wherein the fluid material is a gas.
 15. A method for desulfurizing gases, comprising reacting sulfur to be removed with a compound containing a rare earth element, under reducing conditions.
 16. The method as described in claim 15 wherein the compound is selected from the group consisting of rare earth oxides, rare earth fluorocarbonates, rare earth oxyfluorides, and mixtures thereof.
 17. The method as described in claim 16 wherein rare earth sulfides and oxysulfides, which form by the reaction of sulfur and rare earth-containing compound, are subsequently reacted with oxygen to restore capacity for desulfurization.
 18. The method as described in claim 15 wherein the sulfur to be removed is in the form of hydrogen sulfide.
 19. A method for decreasing the hydrogen sulfide content of gases, comprising reacting at least a portion of the hydrogen sulfide with rare earth oxides to form one of the group consisting of rare earth sulfides, rare earth oxysulfides, and mixtures thereof. 